N-phosphonomethylglycine (glyphosate) is an important broad spectrum herbicide. One conventional precursor to glyphosate is N-phosphonomethyliminodiacetic acid having the following structure: ##STR1## Conventional routes to compound (I) typically involve phosphonomethylating iminodiacetic acid (IDA), the latter obtained by recovering (IDA) from the crude hydrolysate of iminodiacetonitrile (IDAN) by acidification with a mineral acid, crystallization of IDA, filtration thereof, and drying. Such processes result in waste in that the alkali metal salt solution separated from IDA upon the filtration step contains unrecovered IDA. Indeed, fractional crystallization is necessary to precipitate the alkali metal salt, which can then be separated from the IDA solution by centrifugation. In U.S. Pat. No. 3,808,269, disclosed is a process of recovering IDA from an aqueous solution of sodium sulfate and the amino acid by adjusting the pH of the solution to 1.5-3 to form an IDA precipitate and a first mother liquor, and separating and recovering the IDA precipitate therefrom. Sodium sulfate can then be precipitated from the first mother liquor by concentrating the liquor and adjusting the temperature so as to prevent the concomitant precipitation of IDA. The process can then be repeated. However, at some point successive fractional crystallization steps become uneconomical, notwithstanding the presence of additional product in the solution. U.S. Pat. No. 3,852,344 discloses a similar process wherein the sulfuric acid is replaced with hydrochloric acid, with sodium chloride being recovered as the by-product instead of sodium sulfate.
U.S. Pat. No. 5,011,988 discloses an additional process which can be carried out on the waste streams generated from the process of the '269 patent. Thus, the temperature of such waste streams is adjusted so as to precipitate IDA and sodium sulfate decahydrate in the same mother liquor. The mixed crystals are then separated from the mother liquor and recycled to an earlier step in the IDA production process.
As disclosed in U.S. Pat. No. 3,288,846 to Irani, phosphonomethyliminodiacetic acid crystals can be prepared from IDA acid, formaldehyde and phosphorous acid in the presence of hydrochloric acid: ##STR2##
In order to simply the foregoing process, compound (I) can be prepared by hydrolyzing iminodiacetonitrile with an alkali metal base to form an alkalimetal salt of iminodiacetic acid, which is converted to IDA strong acid salt and phosphonomethylated, as disclosed in U.S. Pat. No. 4,724,103. Specifically, the alkali metal salt of IDA is reacted in series first with a strong mineral acid to form the strong acid salt of IDA and the alkali metal salt of the strong acid, and then phosphonomethylated by reacting the strong acid salt of IDA with phosphorous acid and formaldehyde to provide compound (I) and an alkali metal salt. An amount of water sufficient to dissolve the alkali metal salt is added, and compound (I) is separated as a precipitate. Similarly, U.S. Pat. No. 4,775,498 discloses a process for preparing N,N-diacetic acid aminomethylenephosphonic acid by adding phosphorous trichloride to an aqueous solution of a first quantity of an alkali metal salt of IDA and forming a mixture of phosphorous acid and iminodiacetic acid hydrochloride and alkali metal chloride, adding formaldehyde to the mixture while simultaneously adding a second quantity of the alkali metal salt of IDA, adding an amount of water sufficient to dissolve the alkali metal salt, adjusting the pH of the resulting mixture to the isoelectric point of N,N-diacetic acid aminomethylenephosphonic acid, and separating the precipitated acid: ##STR3##
However, alkali metal salt solutions of IDA, such as IDANa.sub.2, are impractical to store and ship on a commercial scale, as the solubility is only about 22% at room temperature and about 40% at temperatures on the order of 60.degree. C. Even where 40% solubility is acceptable, it is expensive and inconvenient to maintain the high temperatures necessary for such solubility.
It therefore would be desirable to produce phosphonomethyliminodiacetic acid (PMIDA) from an intermediate that can be readily stored and/or shipped at ambient temperature on a commercial scale.